Drive mechanisms, such as gearboxes and motors, for actuators, such as valve stems, are frequently characterized by appreciable backlash. In certain instances, the drive of a motor required to take up the backlash can be on the same order of magnitude as a drive command signal derived by an automatic controller. Prior systems developed for compensating the backlash of a drive mechanism have generally been concerned with backlash during normal operation in response to an automatic controller. These systems are generally inappropriate for use in conjunction with drive mechanisms susceptible to automatic, as well as manual, control.
Manual control of a drive mechanism introduces problems because an operator may cause the drive mechanism to be left at an indeterminable position between a pair of backlash limits. It cannot be assumed that a drive mechanism controlled by manual operation is set at one or the other of the backlash limits. Therefore, the amount of drive necessary to compensate for the possible intermediate backlash state is unknown when drive is changed from manual to automatic. Therefore, in many instances the first signal supplied by a controller to a drive mechanism after the controller has been switched from manual to automatic is not of the proper amplitude to activate the actuator to the correct position. The resultant error in the position of the actuator may be continuously propagated with deleterious results, for one or more process control actions. This is particularly true in systems that have only a feed forward loop.